Further to earlier explanations, within the British Army, also used in response to an answer to an obvious question. This is a question that you have set the other person up to answer. Can become quite frustrating as you become paranoid answering any question in case it is a Wah!

Steve "Is that a can of beer in your hand Bill?"
Bill "Yeah!"
Steve "Wah!"
Bill "Bugger, been Wah'd again!"